1. Field of the Invention
The present invention relates in general to fiber reinforced glass-ceramic composites. More particularly, the present invention relates to graphite fiber reinforced lithium-aluminum-silicate composites having near zero coefficients of thermal expansion which are useful in fabricating high energy laser mirror substrates.
2. Description of Related Art
Fiber reinforced composite materials have long been utilized in applications which require high strength, low weight, and specialized thermal properties. Due to their low coefficient of thermal expansion and good thermal conductivity, graphite reinforced composites, in particular, have been investigated for their possible utility in the fabrication of mirrors for high energy laser systems.
High energy laser systems utilize precision mirrors which are aligned with an excited medium. These precision mirrors have highly reflective surfaces for repeatedly reflecting radiation from the excited medium. Effective laser action depends upon a buildup of sufficient energy from these repeated reflections to produce a high energy coherent beam. If the mirrors become misaligned during operation the efficient production of the high energy laser beam can be severely compromised. One persistent cause of mirror misalignment is dimensional change due to the large amount of absorbed thermal energy at the surface of the mirror.
To prevent mirror distortion, most high energy laser systems require heat exchangers in the mirror substrate to remove the absorbed energy and minimize distortion of the optical surface. These heat exchangers, however, result in increased complexity and weight in the form of relatively high density liquid coolant, and the associated pumps and plumbing. Additionally, the flowing liquid coolant is a source of vibration. The coolant transmits pump pulsations into the mirror system and turbulent flow creates additional jitter in the mirrors and their mounts.
One approach to reducing the dimensional changes due to thermal absorption is to utilize a mirror substrate material which exhibits good thermal conductivity (K) and has an extremely low, near zero, coefficient of thermal expansion (CTE). Preferably the mirror substrate has a coefficient of thermal expansion near zero in the plane of the surface of the mirror and a high thermal conductivity perpendicular to the plane of the mirror. Many current laser mirror substrate materials fall short of this requirement. For example, ultra-low expansion titanium silicate glass or fused silica have a low coefficient of thermal expansion but also a low thermal conductivity. Material such as molybdenum, silicon, or silicon carbide are also used for laser mirror substrates but they have a high CTE as well as a high K.
Glass-ceramic composites, such as graphite reinforced lithium-aluminum-silicate (GLAS) composites have been under development for use in applications such as laser mirror substrates. GLAS composites show promise for these applications because they can be designed and fabricated into mirrors having relatively low coefficient of thermal expansion in the plane of the mirror and a high thermal conductivity in the direction perpendicular to the mirror. One class of reinforced lithium-aluminum-silicate composite is described in U.S. Pat. No. 4,791,076. The reinforced composite taught in this patent includes graphite fibers in a matrix of silica, boron phosphate and modified beta-spodumene (lithium-aluminum-silicate.) These have significantly lower coefficients of thermal expansion than both titanium silicate glasses and fused silica. In fact, beta-spodumene undergoes contraction when heated to temperatures of 700.degree. C. or more, exhibiting a negative coefficient of thermal expansion in the range of approximately -0.55.times.10.sup.-6 /.degree. F.
One problem associated with producing GLAS composites having the appropriate thermal characteristic is the highly anisotropic nature of the graphite fiber reinforced material. The coefficient of thermal expansion can vary significantly along the length of the material, thus causing a problem with the predictability of the thermal properties. Additionally, the resulting coefficient of thermal expansion is highly process dependent. Thus, even though the GLAS composites exhibit generally good thermal properties for use in high energy laser optics, they have not advanced to the point where they can be used in the absence of turbulent-flow cooling fluids.
Accordingly, it would be desirable to provide materials which can be fabricated into precision mirrors for use in high energy laser systems without the accompanying use of heat exchangers. In connection with this, it would be desirable to provide glass-ceramic composites having a near zero coefficient of thermal expansion in one plane and a high thermal conductivity in a plane perpendicular to the plane with the near zero coefficient of thermal expansion. It would also be desirable to provide methods for fabricating fiber reinforced composites having near zero coefficients of thermal expansion.